If you're serious about photography you take great care in setting your
camera's exposure. A scanner is also a camera, and a possible weak link in your
production chain unless similar care is taken in setting its exposure. As an
exposure meter is used to monitor a camera's exposure, the histogram is the
primary tool used to monitor a scanner's exposure. That's why histograms are
important.

Histograms are used in scanning to show how the values of the pixels that
have been captured are distributed over the scanner's tonal sensitivity range,
just as a camera's meter indicates how well the image matches the film's tonal
range.
In other words, they tell you how efficiently the scanner is being used. Without them it would be extremely difficult to achieve technically optimal scans.

Histograms provide a graphical display of the state of the image,
summarizing the vast quantity of data contained in a typical scan

It is nearly impossible to make the final adjustments through visual
feedback alone. Monitors are incapable of displaying the subtle differences
in the shadow areas which are easily discerned in a histogram

These principles also apply to digital cameras which display histograms.

These two images illustrate the limitations of the monitor as a scan
diagnostic tool. The picture on the right has more shadow detail than the
one on the left. To view these differences, however, one must increase
monitor brightness and contrast to the maximum -- hardly a convenient way of
comparing image quality. Why should one worry about detail not visible on
the monitor? For a Web application, for example, you may not need
such detail, but in another application you may
need that detail if some manipulation of the tones or colors make these tonal
areas more visible. Or some other output medium that you may want to use,
presently or in the future, may
be capable of displaying these distinctions.

How the Scanning Process Relates to
Image Histograms

fig. 1

The typical scanner has a density
range less than that attainable on film. Any tones denser than that
perceivable by the scanner become black, also known as clipping.

The width of the scanner histogram's horizontal
axis--from the left (black) to right margin (white)--represents the
scanner's density range. Each of the bars, of course, represents the
pixel count for the tone value of its position.

This particular histogram shows
that approximately two-thirds of the scanned image's pixels--the dark gray
area--are in the
darker half of possible tonal values.

The Photoshop version of the histogram for the image is
functionally similar:

The scanner histogram, at left,
is for the preview image, which is based on a small sample of
pixels. The Photoshop histogram is for the scanned image.

One of the primary reasons for
performing a preview scan is to obtain an image histogram. Yet I would
guess that because of simple ignorance the vast majority of non-professional
scanner users simply ignore this extremely useful device. I would also guess the
same applies to users of graphics editing programs such as Photoshop. This
section gives an idea of how to begin to use histograms in scanning.

If you get fig. 2 as an initial preview
histogram, this is about as good as it gets. The image's tonal range neatly matches the
scanner's density range without having to apply curves adjustments. The left of the histogram (b) abuts the base of
the scanner's density range and the right of the histogram (w) abuts the
high-end of the range. The scanner's sensitivity is used at maximum
efficiency: none of the image is outside the scanner's sensitivity range and
samples will be obtained for all 256 tonal values.

fig. 3

Fig. 3 is a histogram where both b and w are in the left-most
and right-most quarters of the histogram but not touching the edges. The
scanner's sensitivity is used efficiently.

Fig. 4a is a histogram, most commonly
encountered in scanning transparencies, where some of the film's tones are
beyond the scanner's sensitivity. Fig. 4b, a notional histogram implied by
fig. 4a, depicts the part of histogram (blue) falling outside the scanner's density
range.
The tones in the blue area are imaged as black.

Figs. 6 and 7 are histograms for scans of the same image without curve
adjustment and illustrate making use of the available density range. Even
if the scan represented in fig. 6 had resulted in an image visually closer to
the desired final image, the image represented by fig. 7 is preferred because it
spans the range of available tone values and as a result contains more
information than 6. With more information, the user has greater
latitude for subsequent manipulations of the image. One could produce the
image in 6 from 7 but not the reverse. Note that although figs. 6 and 7
are for the same image, fig. 7 occupies a greater area.